Comments:

Jitneys do have a lot of uses. But the fact that they use existing roads is both a cost and a benefit - it's a benefit in that you don't need to build new infrastructure, but it's a cost in that they don't ease congestion. In fact, they may worsen congestion, because the jitney may still be driving around even when there isn't a passenger.

It's not clear that they eliminate dependence on transit unions - there's nothing to stop the jitney drivers from organizing together and unionizing. (I would think that any law that bans unionization would be government intrusion in the peaceable assembly of private individuals.)

I'm not sure what you mean by "low cost" - in every city I've been to, a taxi ride is more expensive than the corresponding bus or train ride. It costs more on the supply side because you have to have a separate heavy chunk of metal for every passenger, and you burn gasoline for each individual trip, while buses and trains can achieve economies of scale in transporting dozens or hundreds of people simultaneously. And there's greater willingness to pay on the demand side, because you get directly to where you're going instead of going to a pre-planned station or bus stop and walking the rest of the way, and you don't have to stop at every station on the way.

I'm not entirely sure what the difference is between jitneys and taxis, unless it's something about licensing or working as a solo entrepreneur. I think that taxis are a very important form of public transportation, and in Los Angeles it's the biggest missing link - we've got a great bus system that provides citywide mobility with enough frequency to be dependable; we've got a decent rail system that covers a few of the routes with highest ridership (though it needs to cover more, especially the north-south routes on the west side); but we don't have a taxi system for spontaneously demanded quick diagonal travel. For that, people have to rely on private automobile together with government-provisioned land set aside for vehicle storage at all locations throughout the city.

It seems to me that a functioning city really needs all three - taxis are never going to be able to ease congestion on the densest routes, and they're not as cheap as buses for the medium density all-day demand that exists. But buses and trains can never accommodate the kind of spontaneous diagonal trip that taxis are ideal for.

I think that any plan for a city transportation network that doesn't include all three (as well as accommodations for walking and some other non-motorized transportation) just isn't realistic, unless you're talking about a really small town that has no need for trains anywhere, or a megadense city that needs trains even for the routes that other cities use buses on.

I admit, I hadn't read any of them before, but I've read all of them now. Nothing in any of them contradicts any of my points, except perhaps about the potential distinctions between a jitney and a taxi - and it seems to me that there isn't really a sharp distinction here, but rather a continuum between having a completely fixed fare (like a jitney) and having a completely metered fare (which no taxis have, since they all have a base fare plus a metered fare). I agree that many of the regulations that restrict the supply of taxis and jitneys ought to be repealed in order to improve the transportation systems of our cities. But it doesn't seem to me that this would or should replace either buses or trains in cities, unless they were allowed to take advantage of all sorts of subsidies that buses and trains don't have. For instance, land use regulations provide subsidies to any use of private automobiles by providing free parking everywhere (except a few downtown destinations, where parking is still generally priced substantially below market rate). And, as discussed in the Law and Economics article, in the early days jitneys were allowed to operate without paying for proper insurance and bonds against harm to customers and passers-by.

It's probably true that some lower-demand bus lines would be discontinued if there were better options in terms of taxis and jitneys, but I don't see how jitneys and taxis would ever compete with something like the Manhattan subways, or even the Los Angeles subways, for the routes that they serve, without further government subsidies of vast expanses of asphalt for them to drive on.